1. Field of the Invention
The present invention relates to a honeycomb structure used for a filter for purifying an exhaust gas or the like.
2. Description of the Related Art
As means for removing particulate matters such as fine carbon particles and like particles emitted from an internal combustion engine such as of an automotive engine, there has been employed a method of once trapping particulate matters by using a honeycomb filter and removing the particulate matters by heating or burning them using a heater or a catalyst.
The conventional filter for purifying the exhaust gas employs a honeycomb structure in which, as shown in FIGS. 12 and 13, one end of cells 90 of the ceramic honeycomb structure 9, with both ends, are alternately closed with plugs 95. That is, referring to FIG. 13, the end surfaces 91 on the upstream side are alternately plugged, checkerwise, for example, by plugs 95 at the end of the cell 90. The cells having plugs on the upstream side are left open on their downstream side, and the cells which are opened on their upstream side are closed with plugs on the downstream side. A catalyst is carried on the partitioning walls 98, depending upon the kind of the filter for purifying the exhaust gas.
The above filter for purifying the exhaust gas traps the particulate matters on the partitioning walls thereof when an exhaust gas from an internal combustion engine passes therethrough, and the particulate matters are removed by being heated and burned using a heater or based upon the catalytic action.
However, the above-mentioned conventional filter 9 for purifying the exhaust gas has problems, as described below.
That is, as shown in FIG. 14, particulate matters 88 trapped from the exhaust gas 8 flowing into the exhaust-gas purifying filer are not necessarily removed by burning in good timing, so that they are often deposited gradually on the partitioning walls 98. For example, while the exhaust gas 8 of a low temperature is being emitted from the internal combustion engine, the burning does not take place despite the catalytic action, and the particulate matters 88 are simply deposited. In this case, the pressure loss increases when the exhaust gas 8 passes through the filter, i.e., through the honeycomb structure, and the filter is abnormally heated and may become cracked or melt-damaged by the heat of burning.
Further, the process of plugging cells of the honeycomb structure at the ends thereof requires very cumbersome and laborious manual work, hindering efforts to decrease the cost of production.
The present invention was accomplished in view of the problems inherent in the conventional art, and provides a honeycomb structure capable of suppressing the occurrence of excessive pressure loss caused by the deposition of particulate matters and of being produced at a decreased cost.
According to one aspect of the present invention, there is provided a honeycomb structure comprising at least one main honeycomb structure and sub-honeycomb structures axially disposed, in series, on both end surfaces of the main honeycomb structure, wherein partial plugs are constituted for some of the cells of the main honeycomb structure by facing the intersecting portions of partitioning walls, surrounding the cells of the sub-honeycomb structures disposed adjacent to the main honeycomb structure, with some of the cells of the main honeycomb structure and the arrangements of the partial plugs at one end surface of the main honeycomb structure differs from that at the other end surface of the main honeycomb structure.
The actions and effects of the invention will now be described.
As described above, the honeycomb structure of the present invention is constituted by combining the main honeycomb structure and the sub-honeycomb structures. The sub-honeycomb structures work as partial plugs for partially closing some cells of the main honeycomb structure. The partial plugs at the left end surface of the main honeycomb structure are arranged differently from those at the right end surface of the main honeycomb structure.
Therefore, the honeycomb structure has a construction such that the openings on one end of the cells of the main honeycomb structure, between both ends thereof, are at least partially closed by the partial plugs constituted by the sub-honeycomb structure. When the honeycomb structure is disposed in a fluid, therefore, as the fluid enters into the cells a difference occurs in the flow resistance of the fluid, depending upon the presence of the partial plugs, in the upstream end surface of the main honeycomb structure. Accordingly, the fluid easily enters into the cells without the partial plugs at the upstream end surface thereof.
In the cells having the partial plugs on the downstream end surface of the main honeycomb structure, a condition is established wherein a fluid encounters a smaller resistance, when it flows through a partitioning wall into a cell without a partial plug on the downstream side, than when it flows to the downstream side through the partial plugs on the downstream end surface of the main honeycomb structure. When the honeycomb structure is used, for example, for a filter for purifying an exhaust gas, the exhaust gas, which is the fluid, flows through the partitioning walls at a larger flow rate than when it flows through the main honeycomb structure without plugs, so that it is possible to trap the particulate matters in the exhaust gas by the partitioning walls with improved efficiency.
The plugs constituted by the sub-honeycomb structure are partial plugs which do not fully close the cells of the main honeycomb structure but only partially close them. This suppresses the problem, as explained above, that occurs when the cells are completely closed by the conventional plugs.
That is, when the cells are completely plugged, only a little of the fluid flows through the partitioning walls under an abnormal condition where the particulate matters have been excessively deposited on the partitioning walls, and the pressure in the cells rises excessively. According to the present invention, on the other hand, the cells of the main honeycomb structure are partially plugged, so that even in an abnormal condition where the pressure has been raised in the cells, the fluid flows through a space in a cell left by the partial plugs, and the pressure is not excessively elevated.
Further, the sub-honeycomb structure has the intersecting portions of the partitioning walls facing the cells of the main honeycomb structure to form the partial plugs. Therefore, no cumbersome production process needs to be executed, unlike the conventional plugging process. This makes it possible to produce the honeycomb structure at a decreased cost and more efficiently.
According to the present invention, therefore, it is possible to suppress the occurrence of excessive pressure loss caused by the deposition of particulate matters and to provide a honeycomb structure at a decreased cost.
According to another aspect of the present invention, it is preferable that the ratio of the total length of the sub-honeycomb structures in the axial direction thereof to the length of the whole honeycomb structure in the axial direction thereof, be in a range of from 5 to 60%. When the above ratio is less than 5%, the honeycomb structure has a decreased strength and may become cracked. When the ratio exceeds 60%, on the other hand, the main honeycomb structure possesses a decreased filtering area, so that it has considerably decreased capacity to trap the particulate matters.
According to a further aspect of the present invention, preferably, both the cells of the main honeycomb structure and the cells of the sub-honeycomb structures located at the ends of the main honeycomb structure, have a square shape and are arranged with their sides being inclined by about 45 degrees relative to each other and intersecting. In this case, the intersecting portions of partitioning walls of the sub-honeycomb structures can be regularly arranged relative to the cells of the main honeycomb structure, and hence the partial plugs can be regularly arranged. Therefore, an enhanced effect is obtained by the arrangement of the partial plugs.
According to a still further aspect of the present invention, it is preferable that the main honeycomb structure and the sub-honeycomb structures be joined together with an adhesive. In this case, the main honeycomb structure and the sub-honeycomb structures can be firmly secured together and deviation in the positional relationship between them can be prevented during the use.
According to a yet further aspect of the present invention, a casing is arranged to surround the honeycomb structure, and the arrangement of the main honeycomb structure and of the sub-honeycomb structures is secured by the casing. In this case, it is possible to omit the step of adhering the main honeycomb structure and the sub-honeycomb structures together, and thus to simplify the production process.
According to a still further aspect of the present invention, it is preferable that the main honeycomb structure be made of a ceramic material. As the main honeycomb structure, the partitioning walls thereof can be made of a metal or any other material as long as they can have permeability. Among them, a ceramic material is preferable, because it can easily realize the honeycomb structure having partitioning walls, with pores, exhibiting permeability.
The sub-honeycomb structures may be made of a ceramic material, a metal, etc.
According to another aspect of the present invention, it is preferable that the intersecting portions of partitioning walls of the sub-honeycomb structure have an increased thickness. In this case, the partial plugs exhibit an enhanced effect.
According to a further aspect of the present invention, there is provided a honeycomb structure in which a plurality of honeycomb structures are arranged in series, and partitioning walls of a first honeycomb structure are disposed so as to face at least some of the cells in a second honeycomb structure, and when a fluid flows in the direction in which the plurality of honeycomb structures are arranged in series, the flow resistance of the fluid in a passage formed by a cell defined by the partitioning walls in the second honeycomb structure located on the upstream side of the first honeycomb structure varies from a next passage formed by the adjacent cell, though all cells of the second honeycomb structure.
In the honeycomb structure of the present invention, each flow resistance of the fluid flowing through the cells in the honeycomb structure located on the upstream side is set to differ between two adjacent cells by the above-mentioned combination of the plurality of the honeycomb structures. Hence, a fluid entering into cells having high flow resistances flows through the partitioning walls into adjacent cells having low flow resistances. Therefore, a notable advantage is obtained when the above-mentioned honeycomb structure is used for the filter.
According to a further aspect of the present invention, there is provided a honeycomb structure in which a plurality of honeycomb structures are arranged in series, and partitioning walls of the first honeycomb structure are disposed so as to face at least some of the cells in the second honeycomb structure, and when a fluid flows in a direction in which the plurality of honeycomb structures are arranged in series, the density of cells of the second honeycomb structure located on the upstream side is higher than the density of cells of the first honeycomb structure located on the downstream side.
In the honeycomb structure of the present invention as described above, the density of cells in the honeycomb structure on the upstream side is higher than that of the honeycomb structure of the downstream side. Accordingly, the open surfaces of the cells of the honeycomb structure on the upstream side, which include the portions facing the partitioning walls of the honeycomb structure on the downstream side and the open portions, can be easily formed. It is therefore possible to set the flow resistance of the fluid flowing through a cell in the honeycomb structure located on the upstream side so as to differ from the adjacent cell thereof. Hence, the fluid entering into a cell having high flow resistance flows through the partitioning walls into the adjacent cells having low flow resistances. Therefore, a notable advantage is obtained when the above-mentioned honeycomb structure is used for the filter.
According to a further aspect of the present invention, the above-mentioned plurality of honeycomb structures can be arranged with a gap, which is not larger than 10 mm, between two adjacent honeycomb structures. When the gap exceeds 10 mm, the effect of adjusting the flow resistances between the adjacent cells on the upstream side is not achieved to a sufficient degree despite arranging the plurality of honeycomb structures in series.